1. Field of the Invention
The present invention relates to a method of applying an electrical stress to a low-temperature poly-crystalline thin film transistor, and more particularly, to a method of applying an electrical stress to a low-temperature poly-crystalline thin film transistor for enhancing an electrical stability and characteristic by applying an electrical stress to the low-temperature poly-crystalline thin film transistor which has been crystallized using a laser crystallization method or a metal induced lateral crystallization method.
2. Description of the Related Art
In general, a thin film transistor which is used in a display device such as a liquid crystal display (LCD) or an organic light emitting diode (OLED) is fabricated by processes of depositing silicon on a transparent substrate such as glass or quartz, forming a gate insulation film and a gate electrode, injecting a dopant into a source and a drain, and then annealing the dopant-injected source and drain to thereby activate the same and to then form an insulation layer. An active layer forming a source, a drain and a channel of a thin film transistor is formed by depositing a silicon layer on a transparent substrate such as glass using a CVD (Chemical Vapor Deposition) method. In this case, a silicon layer directly deposited on a substrate by the CVD method and so on is an amorphous silicon film having a low electron mobility.
By the way, as a display device using a thin film transistor gradually requires for a fast operating speed and becomes compact in size, a degree of integration of a driving integrated circuit (IC) becomes large and an aperture ratio of a pixel region becomes reduced. Accordingly, an electron mobility on a silicon film should be heightened so that a driving circuit is formed simultaneously together with a pixel thin film transistor (TFT), and an aperture ratio of each pixel should be heightened. For this purpose, a technology of heat-treating an amorphous silicon layer and crystallizing the same into a poly-silicon layer of a poly-crystalline structure having a high electron mobility is being used.
A thin film transistor (TFT) made of a poly-crystalline thin film is a well known device, and is fabricated by forming a semiconductor thin film such as silicon on a semiconductor substrate on which an insulation layer has been formed, or on an insulation substrate such as glass. The thin film transistors are used in a variety of integrated circuits, in particular, a switching device which is formed on respective pixels on a liquid crystal display, or a driving circuit formed in a peripheral circuitry portion.
In order to obtain a poly-crystalline thin film which is used for the above-described thin film transistor, amorphous silicon deposited as is well known should be heat-treated at a temperature of 600° C. or higher. However, since a poly-crystalline silicon thin film transistor should be formed on a glass substrate as a device for driving a liquid crystal display, a heat-treatment temperature should be low at 600° C. or lower which is below a temperature at which the glass substrate is deformed. Thus, to solve the problem, the following two methods are under study.
According to a first method, laser is irradiated on a silicon thin film and part of the silicon thin film is dissolved so that the silicon thin film is crystallized. In this method, the temperature of the substrate is not made high but only part of the silicon thin film is heated. Accordingly, crystallization can be achieved without deforming the substrate, but problems are exposed in view of uniformity of crystallization, a high production cost, yield, and so on.
According to a second method, metal such as nickel (Ni), palladium (Pd), gold (Ag), and aluminum (Al) is made to contact amorphous silicon. or the metal is injected into the silicon. In this case, amorphous silicon is changed in phase into poly-crystalline even at a low temperature of 200° C. or so. This phenomenon is called MIC (Metal Induced Crystallization). In the case that a thin film transistor is fabricated using a MIC phenomenon, metal remains in the poly-crystalline constituting an active layer in the thin film transistor, to thereby cause current leakage to occur in a channel portion.
Recently, a method of crystallizing a silicon layer has been proposed using a MILC (Metal Induced Lateral Crystallization) phenomenon in which silicide produced by reaction of metal and silicon continues to propagate laterally to sequentially induce crystallization of silicon (S. W. Lee & S. K. Joo, IEEE Electron Device Letter, 17(4), P. 160, 1996). In order to create the MILC phenomenon, heat treatment should be performed at a temperature of 400-600° C.
Methods of fabricating low-temperature poly-crystalline thin film transistors using the conventional MILC method are disclosed in U.S. Pat. Nos. 6,197,623 and 6,689,647.
However, in the case that a thin film transistor is fabricated using the laser crystallization method or the MILC method, a transistor is completed at a low temperature. Accordingly, unstable electrical properties are exposed in the transistor. That is, high leakage current, threshold voltage shift, and so on occurs as the unstable electrical properties. Thus, in order to commercialize the thin film transistor, a technology of electrically stabilizing the thin film transistor is required at the time of fabricating the thin film transistor.